SuperNewb

Hello I am looking for some guidance on programming and using the Uno R3 board.What I want to achieve is for automotive 12-13v usage, however I want it to read a 0-5v sensor.In that 0-5V range I want the circuit to open or close at adjustable set voltages ie. closed at 0-4V open at 4-5V. Of course it can't draw any amperage from the sensor itself as that would affect the reading.Can anyone please point me in the right direction for doing this?

Well an arduino can read a analog voltage in the range of 0-5vdc via it's analog input pins. And a digital output pin plus a transistor can be used to switch a relay on and off. And your sketch software coding can decide at what analog input voltage value to switch the relay on or off, so it would appear to be a simple project for an arduino board?

What transistor will I need to get and could you recommend what thread to read on regard to the software aspect of this? Keep in mind i'm a total newb and need step by babystep guidance :O

Transistor type is determined by the relay type used. Software will be apparent as you learn to program the arduino, there are lots of simple example sketches you can load from the IDE and read to learn what all the commands do and how to use them.

Perhaps the tutorials would be a good place for you to start: http://arduino.cc/en/Tutorial/Foundations

What is the link between the relay used to determine the transistor used? Is the relay merely for powering on the unor3 board?

Your use of language here is confusing. Relays don't power anything. The are electrically operated switches.

If you wish to turn on and off a 12V "thing" with your Uno, then the Uno would need to operate a relay. It probably needs a smallish transistor to give it enough oomph to turn on the relay coils. Then the 12V goes through the relay contacts, not touching the Uno in any way. You would need a diode across the relay coils in this case. Google "arduino relay" or similar to get lots of ideas about how to do that (over 1.5 million hits).

You can also get solid-state relays (SSRs) from eBay and other places. These can be operated directly from a digital signal, and can switch quite large amounts of current / voltage.

Please post technical questions on the forum, not by personal message. Thanks!